This invention relates to a fluidized bed heat exchanger, and more particularly, to a vapor generator which consists of a plurality of stacked fluidized beds for generating heat.
The use of a low-grade solid fuel, such as coal, is a well-known source of heat in the use of generation of steam. In some of these arrangements the fuel is disposed in a fixed bed with a chain grate stoker or the like utilized to promote its combustion, and water is passed in a heat exchange relation thereto to produce the steam. However, these arrangements suffer from several disadvantages including problems in handling the solid fuel while adding it to or removing it from the beds during operation. Also, a relatively low heat transfer is achieved and the bed temperatures are often nonuniform and hard to control.
Attempts have been made to utilize a fluidized bed to produce heat for generating steam due to the fact that a fluidized bed enjoys the advantages of an improved heat transfer rate, a reduction in corrosion, a reduction in boiler fouling, a reduction in sulfur dioxide emission, a relatively low combustion temperature and a reduction in boiler size. In these arrangements, air is passed upwardly through a mass of particulate fuel material causing the material to expand and take on a suspended or fluidized state. However, there is an inherent limitation on the range of heat input to the water passing in a heat exchange relation to the fluidized bed, largely due to the fact that the quantity of air supplied to the bed must be sufficient to maintain same in a fluidized condition yet must not cause excessive quantities of the fuel material to be blown away.
This disadvantage is largely overcome by the arrangement disclosed in U.S. Pat. No. 3,823,396 issued to Bryers and Shenker on July 16, 1974, and assigned to the same assignee as the present application. In the arrangement disclosed in the latter patent, the furnace section of the generator was formed by a plurality of vertically aligned chambers, or cells, each containing a fluidized bed. The fluid to be heated was passed upwardly through the fluidized beds in a heat exchange relation thereto to gradually raise the temperature of the fluid. A tube bundle was placed in the area above each bed to provide a convection surface for the effluent gases from each bed.
However, in this type of arrangement the volume of space available above each bed to receive the convection surface is relatively small due to limitations placed on the cross-sectional area of each cell caused by tube spacings, welding accessibility, dust clogging, combustion requirements, etc. As a result, the convection surface defined by the tube bundles was limited to an extent that the mass flow of the effluent gases per area of convection surface and the resulting heat transfer coefficient above each bed, was far less than optimum.